A conventional female clinch fastener, or clinch nut, is permanently installed into a portion of a metal sheet so that the nut becomes an integral part of the metal sheet. The clinch nut is installed by cold-forming the metal of the sheet or the nut, rather than welding. The clinch nut thus provides a threaded bore with greater holding capacity than could be obtained by simply threading an opening in the sheet metal alone. Such a clinch nut is commonly used in automobiles to provide a means for fastening bolts to a metal sheet, for example.
One conventional clinch nut is available from RB&W Corporation of Mentor, Ohio, under the designation SPAC.RTM.nut. The SPAC.RTM.nut includes a back-tapered polygonal portion that is press fit into a slightly undersized opening in a sheet. The nut is locked into the sheet by a coining action as the sheet material flows to fill the space defined by the back-tapered portion. Similar to the SPAC.RTM.nut are the STRUX-nut offered by Maclean-Fogg, and the HT-nut offered by NPR. Each of these clinch nuts relies upon the volumetric displacement of parent material from the sheet to fill an undercut or back-taper to create a kind of dove-tail joint between the fastener and the sheet. Unfortunately, such a clinch nut may not have sufficiently high performance characteristics, such as pull-through, push-out, and torque-out.
Another conventional clinch nut offered by the assignee of the present invention, requires that a hexagonal opening be punched into the sheet to accommodate the hexagonal body portion of the clinch nut. In other words, the clinch nut is a directional nut which requires proper orientation of the opening and proper alignment of the nut within the opening.
Another conventional clinch nut known commercially as the FLANGEFORM.TM. from RB&W, is disclosed in U.S. Pat. No. 4,018,257 to Jack entitled Self-Flanging Nut and Joint Construction. The FLANGEFORM.TM. nut forms, and then wraps around, a collar or pilot in the sheet into which the nut has been pressed. The nut is press fit into an undersized opening in the sheet thereby bending edge portions of the sheet downward to form the pilot for receiving the tubular body of the nut. A laterally extending flange on an end of the tubular body is seated flush with the surface of the sheet. The tubular body includes a threaded bore over a first portion adjacent the head. A second portion of the tubular body includes an enlarged bore which facilitates folding of side walls of the tubular body over and radially outwardly to form a second or locking flange to secure the nut within the pilot of the sheet. A relatively small number of circumferentially distributed ribs extend longitudinally along the first portion of the tubular body to prevent rotational movement or torque-out of the nut.
The FLANGEFORM.TM. nut is a nondirectional nut, that is, it can be installed in a circular opening without concern for relative rotational orientation of the nut and the opening. In addition, this nut offers improved performance characteristics over those conventional nuts described above which rely on the volumetric displacement of parent material from the sheet.
Unfortunately, the FLANGEFORM.TM. nut suffers from a number of disadvantages including a relatively high cost of manufacture due, at least in part, to its varying cross-sectional shape. Moreover, for a given nut size, only a limited range of sheet material thicknesses and sizes of openings can be accommodated. In other words, since a predetermined portion of the tubular body is folded over for a given nut size, the length of the pilot that can be effectively clamped between the head and the folded over locking flange must be accurately controlled.
A further drawback of the FLANGEFORM.TM. nut is that it cannot readily be made to have a so-called "prevailing torque" feature. A prevailing torque fastener includes slightly distorted threads that serve to lock the mating fastener in position and prevent unintended loosening of the fastener. Because the FLANGEFORM.TM. nut includes a threaded bore extending only partially through its lengthwise extent, the prevailing torque feature is not available.
The FLANGEFORM.TM. nut is also secured by the second metal flange which is formed by bending or folding over a portion of the tubular body to abut end portions of the pilot formed in the sheet. This type of bending to cold-form the locking flange produces a relatively weak metal flange that may produce a nut having reduced pull-out performance.
Similar to the FLANGEFORM.RTM. nut, is the PIERCEFORM.RTM. stud offered by Kean Manufacturing of Dearborn Heights, Mich. The PIERCEFORM.RTM. Stud is a male clinch fastener, or clinch bolt, having a threaded stud extending outwardly from a head, the head, in turn, being secured within a pilot formed in the sheet as described above similar to the FLANGEFORM.TM. nut. A portion of the head is a hollow tube having walls that are folded over to form the locking flange. Accordingly, the PIERCEFORM.RTM. Stud suffers from similar disadvantages as the FLANGEFORM.TM. clinch nut.